A THREE-PHASE THREE-SWITCH TWO-LEVEL PWM RECTIFIER Deivis Borgonovo, Yales RBmulo de Novaes, Ivo Barbi (Senior Member, IEEE)
Federal University of Santa Catarina - Department of Electrical Engineering - Power Electronic Institute E-mail:
[email protected] [email protected] [email protected] Abstract - This paper presents a new topology for a high power factor three-phase P W M rectifier system, without a neutral wire, with output voltage control and high efliciency. The control is simple and was implemented using commercial single phase modulators with independent current loops and a voltage loop. The power circuit is also very simple, permitting the use of low cost power devices. A mathematical analysis for the converter will be presented, including the determination of all the power devices, the current and voltage transfer functions and a project procedure. Finally, results obtained through digital simulations and the experimental results obtained from a 6kW prototype will he presented. 1. INTRODUCTTON
Three-phase AC-DC converters without neutral wires are used in many applications, such as telecommunications power supplies, UPS’S and electric drives. Conventional circuits, using thyristors and diodes with passive filters, despite their simplicity and reliability, do not comply with intemational current hamonic standards. Therefore, efforts have been made by engineers to develop the so-called PWM rectifiers, capable of drawing practically sinusoidal current from the mains. Many different PWM three-phase topologies featuring low input current THD, most of them belonging to the family of three-level converters, have recently been proposed in the literature.
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These converters require a complex strategy to balance the voltage across the output filter capacitors. The circuit introduced in this paper does not need a output capacitor mid-point connection to work properly. Therefore, it is much simpler to design and control than its three-level counterpart, However, all the remaining desirable features are preserved, presenting unity power factor with low THD and output voltage control. 11. STRUCTURE AND CHARACTERlSTlCS
The proposed circuit for the high power factor PWM three-phase rectifier is presented as a natural evolution of the most widely used topology to control the input current of single-phase rectifiers, the boost converter in continuous conduction mode (CCM), presented in the top left-hand comer of Fig.2. The development of the three-phase PWM rectifier’s basic structure is presented in F i g 2 Fig2 presents three independent single-phase rectifiers with output capacitors large enough to maintain the output voltage practically constant. However, the presence of a neutral point is undesirable. The neutral wire can be removed and the converter will still work properly, even though the topological stages are different. The proposed circuit for the three-phase PWM rectifier is presented in Fig.1:
Fig.1: Proposed circuit for the three-phase P W M rectifier.
0-7803-7754-0/03/$17.00 02003 IEEE
1075
From the original shucture, the boost inductor can he moved to
the input, adding yet a new boost diode, with no substantial modification on the converter. Now, gathering three single-phase rectifiers supplying the same load, it can be obtained the circuit presented below:
Then, maintaining the same circuit, but drawing it in a different way.
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I I It can be noted that the three-phase converter presents an interesting symmetry. The input current period can be divided into 6 sectors, and in each sector .the converter works symmetrically. So, the analysis can he performed for a chosen sector and be then easily extended to the others. Each sector is defined by the line that presents the highest absolute value of the input current, defined by the sum of the absolute values of the other two input currents. Therefore, the topological stages and all the theoretical analysis for the converter will be presented for one sector. So, initially, the input voltages are given by: V, (1) = Vp .sin(w .t)
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V,(t) = Vp .sin(w.r-120”) & ( t ) = Vp.sin(w, t + 120O)
(1)
Considering that the input currents are images ofthe input voltages, thus the sector defined by: 6Oa
